In recent years, solder resists are used as one of protection films for the circuit surface during printed wiring board production. For high packing density, solder resists have been increasingly required to have both high fine patterning capability and high positional accuracy. As a protection film production method that can meet the above requirements, photolithographic patterning using photosensitive resin compositions is widely used. Examples of such photosensitive resin compositions are epoxy-based compositions, which have been widely used as solder resist materials for protection films on metal interconnections (see e.g., Patent Documents 1 and 2).
However, as conventional materials used as sources of protection films contain large amounts of inorganic filler such as barium sulfate or silica, the films produced from these materials have the disadvantages of being less flexible and thus unable to be subjected to bending processing.
To solve this problem protection films are produced using two different materials during printed wiring board manufacture, where a film called “cover lay” is applied to bending areas and a photosensitive solder resist is applied only to patterning areas. A “cover lay” refers to a polyimide film coated with an adhesive. While many of the solder resists are prepared by coating of printed wiring boards with liquid solutions, adhesive-coated polyimide films are folded with a mold and then attached to printed circuit boards with a pressing machine. Thus, with this method, problems like yield reduction occurred due to involvement of two different steps, thus resulting in demand for solder resists which are capable of fine patterning while ensuring flexibility and heat resistance.
Moreover, from the viewpoint of environment protection, recycling of finished products has been strongly encouraged. In this connection, products are strongly desired that can prevent generation of polluting chemicals during the recycling process. However, the current situation is that resin materials used as cover lay materials and interlayer insulating materials for flexible printed wiring boards require sufficient flame retardancy and, therefore, for flame retardancy, many of the resin materials need to contain either halogenated compounds (e.g., brominated aromatic compounds) which will produce dioxins when burned, or toxic antimony compounds. Therefore, halogen- and antimony-free materials with excellent flame retardancy and flexibility as well as high insulation reliability are strongly desired.
Photosensitive cover lay materials for flexible printed wiring boards which can meet the above requirements have been suggested (see e.g., Patent Documents 3-5). Nevertheless, neither of the disclosed materials can sufficiently meet the recent stringent requirements for flexibility and insulation reliability.    Patent Document 1: International Application Publication No. WO02/24774    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-300785    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-029702    Patent Document 4: Japanese Patent Application Laid-Open No. 2005-283762    Patent Document 5: Japanese Patent Application Laid-Open No. 2006-251715